Direct answer:
The function of a sphincter muscle is to regulate the flow of substances through a body passage.
Step-by-step explanation:
Sphincter muscles are circular muscles that surround a body passage, such as the esophagus, stomach, or anus. They can contract to close the passage, or relax to open it. This allows the sphincter to control the flow of substances through the passage, such as food, liquids, or waste.
For example, the sphincter muscle at the bottom of the esophagus helps to prevent food from backing up into the mouth. The sphincter muscle at the top of the stomach helps to prevent stomach acid from backing up into the esophagus. And the sphincter muscle at the anus helps to control the release of stool.